We have already seen, while examining the construction of the skeleton, that the bony framework of the wing essentially resembles that of the human arm, and that the limb when stripped of its feathers is no more adapted for flying than our own. The needful expansion is obtained altogether by the addition of the quill feathers, which, as explained in a preceding page, combine in their structure all the qualities requisite for the intended purpose, lightness, firmness, strength, elasticity, and extent of surface: the central part of the arm or wing forms merely a basis of support, into which the quills and other appendages to the wing are securely implanted. It will therefore be easily understood that the importance of the individual quills as instruments of flight will depend very much upon the position they hold in the wing, of which they form so considerable a portion, and, consequently, that they have received names expressive of their relative efficiency. Those that are affixed to the bones representing the hand (Fig. 12, _p_, _q q_), by the length of their stroke, and the peculiarity of their arrangement, are obviously of primary importance, both from their size and the situation they occupy, and have consequently been named the "_primaries_," or the "_primary quills_," (Figs. 2, _f_; 13, 35); they might be called with equal propriety the "_hand-quills_," a term more particularly expressive of the parts to which they are attached. Upon the relative length and other proportions of the primary quills the shape and mechanical power of the instrument principally depend; if the first primary be the longest, the termination of the wing is sharp and pointed (_acuminate_), as in most birds that are remarkable for the swiftness of their flight; whereas if the second, third, or fourth of these quills should exceed the others in this respect, the wing becomes more and more rounded (_obtuse_), and the perfection of its action visibly deteriorated. The "_Secondary Quills_," or "_Secondaries_" (Figs. 2, _c_; 13, 34), are exclusively sustained by the bones of the fore-arm (Fig. 12, _m_, _n_); from their situation being much nearer to the shoulder-joint than the preceding, the extent of their sweep is more limited, and their stroke much feebler; they are, consequently, as their name indicates, of secondary importance in locomotion. [Illustration: _Plate 4 Cassell's Book of Birds_